1. Field of the Invention
The present invention pertains to filtration modules and the like containing semipermeable membranes for separating one component from a fluid mixture of the one component and a second component, such as separating one liquid from another liquid or suspended solids, or a gas from a mixture of gases.
2. Description of the Prior Art
Reverse-osmosis and ultrafiltration membranes for the separation of a liquid from a second component, such as another liquid or suspended solids, have been known for some time. A variety of semipermeable membranes have been developed for both reverse-osmosis and ultrafiltration processes, whereas others have been successful in separating specific gases from gaseous mixtures. While such membranes have proven generally satisfactory in achieving separation, they must be incorporated into a filter module of some sort to provide a practical realization of the membrane performance. Several shortcomings of conventional filtration modules have been noted, some of which result from a particular combination of the semipermeable membrane and other materials layers that are adjacent to, and sometimes in contact with, the membrane.
Permeate carriers withdraw the desired permeate from the low-pressure side of the semipermeable membrane. In general, two areas where improvements have been sought in such filtration systems are in the throughput, or rate of collection, of the permeate and in the maintenance interval or service life of the filtration equipment, which areas may be closely related to each other in certain aspects. For example, the throughput or collection rate of the permeate is directly related to the pressure applied across the semipermeable membrane. In general, the higher the applied pressure, the faster the permeate is collected on the low pressure side of the membrane. However, applied pressures cannot be arbitrarily increased, because semipermeable membranes in general use today do not have a physical structure capable of withstanding very high pressures. That is, upon the application of excessive pressure to certain semipermeable membranes, a rupture of the membrane is experienced, with resulting destruction of the filtration module, as well as contamination of the permeate collected. Elevated pressures are employed not only to force the permeate through the material, but also to drive the permeate along its return path to a collection point, despite what is often referred to as side pressure loss, i.e., the resistance to flow of the permeate along or through a permeate carrier.
Even if a rupture of the membrane material is not experienced upon application of elevated pressures, membrane performance can be severely degraded due to sagging, folding or bunching of the material in the filtration device, which can arise principally from a stretching of the membrane material as higher working pressure is applied. Obviously, if the stretching is allowed to continue uncontrolled in a membrane that is not spatially confined, the membrane material will be weakened to the point of rupture. However, short of rupture, the membrane material can sag or bunch, increasing the resistance to permeate flow therethrough. In general, it is desirable to maintain the semipermeable membrane in a taut condition, so as to minimize the thickness of the semi-permeable membrane material through which a permeate is forced to travel.
In addition to providing physical support for the semipermeable membrane so as to prevent its stretching, sagging or rupture, additional layers of material are associated with a membrane in a typical filtration module to provide a return path for the permeate to a collection point. For example, some filtration systems in widespread use today have three layers, including a semipermeable membrane layer, supported upon a mechanical reinforcing or anti-bagging layer and a third layer providing a return path for the permeate. A popular permeate carrier in use today is comprised of Tricot material, which generally may be described as an epoxy or Melamine-coated polyester that has been woven.
Although Tricot material has frequently been a popular choice as a permeate carrier, other carrier materials have continued to be investigated in a search to find even better materials, e.g., having even lower resistance to liquid flow. For example, non-woven polypropylene carriers have been investigated, but have not proven entirely satisfactory, as they have been observed to lack the adhesion to the semipermeable membrane necessary to prevent bunching or sagging of the membrane material. Bunching of stretched semipermeable membranes causes the rapid appearance of locations where accelerated fouling occurs. For these reasons, commercial filtration manufacturers have relied on Tricot, either felt or woven material, to provide the backing and support of the membrane. A particularly advantageous combination of materials forming an integrated filtration system includes a polysulfone membrane, typically 4 mils in thickness, cast on a felt backing material also 4 mils in thickness, and a Tricot material such as K-1015 Hornwood Tricot having a 48 wale rating, i.e., 48 threads per inch. In addition to the polysulfone semipermeable membranes, another type of membrane has been suggested as having useful characteristics for separating oil-water mixtures. The other membrane material is manufactured by W. L. Gore & Associates, Inc., and is commonly referred to as "Gore-Tex." U.S. Pat. No. 3,953,566 discloses a process for producing the material, and suggests several possible filtration applications. Among the filtration applications listed in the patent is the separation of kerosene from a kerosene-water mixture, as an example of a separation of fluids that wet tetrafluoroethylene polymers from non-wetting fluids. More advantageous commercial embodiments using this membrane are sought, particularly ones which will prove themselves in a commercial environment.